1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive that compensates for the bias imparted by a flex circuit cable during a seek operation by utilizing a dual bias curve estimation scheme.
2. Description of the Prior Art and Related Information
Today, computers are routinely used both at work and in the home. Computers advantageously enable file sharing, the creation of electronic documents, the use of application specific software, and electronic commerce through Internet and other computer networks. Typically, each computer has a storage peripheral such as a disk drive (e.g. a hard disk drive).
A huge market exists for hard disk drives for mass-market computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive should embody a design that is adapted for providing rapid access to data and providing high data storage capacity. Satisfying these competing constraints of rapid access to data and high data storage capacity requires innovation in many of the numerous components of the disk drive.
Typically, the main assemblies of a hard disk drive are a head disk assembly (HDA) and a printed circuit board assembly (PCBA). The head disk assembly includes an enclosure including a base and a cover, at least one disk having at least one recording surface, a spindle motor for causing each disk to rotate, and an actuator arrangement. The PCBA generally includes circuitry for processing signals and controlling operations in the disk drive.
An actuator arrangement that is commonly used in hard disk drives is a rotary actuator arrangement included as part of a head stack assembly (HSA) that includes a collection of elements of the head disk assembly. The collection typically includes certain prefabricated subassemblies and certain components that are incorporated into the head disk assembly. For example, a prefabricated head stack assembly (HSA) may include a pivot bearing cartridge, a rotary actuator arrangement, and permanent magnets and an arrangement for supporting the magnets to produce a magnetic field for a voice coil motor.
The rotary actuator arrangement of the HSA may also include a coil forming another part of the voice coil motor, an actuator body having a bore through it, and a plurality of arms projecting parallel to each other and perpendicular to the access of the bore. The rotary actuator arrangement of the HSA may also include head gimbal assemblies (HGAs) that are supported by the arms. Each HGA includes a load beam and a head supported by the load beam. The head is positioned over a track on a recording surface of the disk to write or read data to or from the track, respectively. A flex circuit cable typically connects the processing circuitry of the PCBA to the rotary actuator of the HSA in order to deliver commands such as read and write commands, as well as seeking and tracking commands.
Because of the competitive pressure to continually develop hard disk drives that provide faster and more robust access to data, techniques are continuously being developed to decrease the access time to data. One technique to accomplish this is by increasing the speed and accuracy by which the HSA is pivoted to position the head over a track of the disk in order to access data as part of a seek operation. Particularly, in order to provide faster access to data, techniques are continually being developed to decrease seek time to data.
Seek time generally refers to the time required to move the head of the actuator from a current position or current track to a target position or target track. The seek time is determined by the mechanical characteristics of the HSA and by a seek profile. The seek profile defines, during the time that the actuator is accelerated and decelerated en-route to the target track, a desired acceleration/deceleration profile and desired velocity profile to achieve an efficient and predictable arrival of the head to the target track of the disk. Disk drive makers work diligently to improve the efficiency and accuracy of the seek operation.
One common problem associated with seek operations is that the flex circuit cable typically imparts a bias force upon the actuator that needs to be accounted for in the final control effort to cancel its effect. If the bias imparted by the flex circuit cable is not properly accounted for in the control effort of the seek operation, the seek operation performs poorly resulting in decreased data retrieval time and overall degraded throughput performance.
Particularly, the flex circuit cable typically imparts a spring action that operates upon the actuator as a bias force. Also, other factors affect the bias of the flex circuit cable on the actuator such as windage on the flex circuit cable and actuator, the internal ambient temperature of the disk drive, as well as other factors.
Previous attempts have been made to address flex circuit cable bias in seek operations. For example, U.S. Pat. No. 5,872,674 issued to Eddy discloses a specialized seek controller and tracking controller to attempt to account for various biases. The tracking controller of Eddy is preferably implemented as a digital signal processor (DSP) and includes a proportional integrator differentiator (PID) controller. In conjunction with the DSP tracking controller, Eddy discloses the use of a look-up table that is provided to store predicted bias values and locations addressable by seek direction, previous seek direction, seek length, and head position.
Other more robust techniques have been implemented in modern microprocessor-based disk drives to attempt to compensate for bias forces imparted by the flex circuit cable during seek operations, such as, by storing bias values representative of a single bias curve that is used to estimate bias effects. Unfortunately, these techniques do not provide the desired degree of accuracy for high performance disk drives that have an ever-increasing radial density of data tracks.